Oil Filler Cap Blows Off!

I've owned a 1989 911SE Carerra for 5 months. I noticed an oil/gas smell inside recently when I turned the heater on. Investigations highlighted that the oil filler cap had "blown" off. When screwed back on, it pops off again under acceleration. It is the original cap. Has anyone else had this problem?

Help...how do I stop this happening?

Neil

Hopefully youv'e checked to see if the cap is stripped or cracked. Does it tighten down ok?
If its not a cap problem or the threads on the neck then you must have a real problem. Maybe someone else knows whats really happening. I don't think its common at all though.

Mike Miller
89 911

I don't know if a 89 has a hose to your air box (vent) but this could be blocked or clogged not removing pressure in the tank

good luck
Tim in Sac

do a leakdown test. you may have an internal problem that pressurizes the crankcase. how's it run? does it smoke? do a tailpipe emiss. test for high HC. are any rockers real ticky after a good adjustment? (guide problem). hold your palm tightly on the filler neck and rev it up. is there a lot of pressure?

Thanks everyone. I forgot to mention:-

1. The car is immaculate and runs very well, with no smoke.

2. That when I tighten the cap down, the rubber seal grabs tight and then the cap pops open when extra pressure is applied - a bit like a stripped thread would but the cap looks perfect as does the thread.

3. If I wiggle the cap when it's on the thread it feels a little lose (about 1mm clearance) but I don't know if that's normal or not - can you please advise?

I'm thinking that the best option is to buy a new cap and try that but I'll await some more wisdom from more experienced owners first.

Regards Neil